[X86][Skylake] Replacing -mcpu=skx by -mattr in a codegen test. NFC.

NFC.
Replacing -mcpu=skx by -mattr in the run command of the codegen test: avx512-gather-scatter-intrin.ll.

Reviewers: delena
Revision: https://reviews.llvm.org/D37799
llvm-svn: 313144